The invention relates to cold extrusion of a metal part having a configuration consisting of a cylindrical exterior and a hollow polygonal interior.
1. Field of the Invention
The field of the invention is a new method of applying reverse extrusion by two or more successive extrusion steps whereby a mass of cold metal is first formed into an intermediate preparatory configuration. The preparatory blank is subsequently formed into the final desired configuration having walls of variable thickness circumferentially of the longitudinal axis of the final extruded product.
2. Description of the Prior Art
A method of forming a cup-shaped article having a smooth cylindrical interior surface and an outer polygonal exterior surface is shown by my U.S. Pat. No. 2,904,173 dated Sept. 15, 1959, in which the resulting exterior polygonal surface has its apices modified from a true polygon in order to improve the extrusion process by materially reducing the tendency for uneven flow during the cold extruding operation. In said patent, the original starting billet is converted into the final polygonal product by a single extrusion step and the efficiency of the process is enhanced by eliminating the sharp corners of the exterior polygonal shape. Conversely in the present invention, a method has been devised in which the final product retains the sharp contours of the polygon and improves the efficiency of the extrusion by the design of an intermediate with an unusual shape which in the second step compensates for the metal flow into the sharp corners of the interior polygonal surface. Thus in the manufacture of a part with cylindrical outside and polygonal inside, the new method uses the two step process hereinafter described with intermediate formation of a preparatory blank which is subsequently extruded by the second step.
In the conventional one step prior art method of forming a part with internal polygonal form, the method requires forcing a polygonal punch into the initial cold metal billet with the necessary force required to extrude the part. The force required is substantially higher than the new two step process because the load is in direct proportion to area of the tool and material to be extruded. As an example, using an hexagonally shaped punch, the load required to extrude the part would be the area of the punch multiplied by the unit loading required to extrude the material. Assume for example, that an hexagonally shaped punch is of such size as to have an area of one square inch and is used to cold extrude a metal billet which requires 150 tons per square inch of unit force. This would result in an extrusion force of 150 tons. Thereby, subjecting this hexagonal punch to a load of 150 tons. The new method of the present invention requires only a force of 48 tons, a reduction of 68%. In addition, the prior art one step process results in uneven ends which increases the difficulty in subsequent machining operations to produce the finished article.